\begin{tikzpicture}[
    node distance = 2cm and 2cm,
    N/.style = {circle, draw, font=\Large, 
    minimum size=1.1em},
    every edge/.style = {draw, ->, semithick, shorten >=2pt},
    every edge quotes/.style = {auto=right, inner sep=2pt}
    ]
    \node [N] (2) at (0,0) {2};
    \node [N] (3) [right of=2] {3};
    \node [N] (5) [right of=3] {5};
    \node[] (占位) at ([xshift=-2em]2) {};
    \path (2) edge [->, blue] node [above] {$\varepsilon$} (3)
    (3) edge [->] node [above] {c} (5)
    (3) edge [loop above] node [above] (b) {b} (3) %循环
    (占位.west) edge [->] node [above] {a} (2.west);

\end{tikzpicture}
